1. Technical Field
The present disclosure relates to a light emitting display and, more particularly, to the structure of a light emitting pixel and a driving method thereof, and an apparatus and method for driving the light emitting pixel.
2. Discussion of Related Art
Recently, amorphous-Silicon (a-Si) backplane technology or poly-Silicon (poly-Si) backplane technology has been used for an active matrix organic light emitting diode (AMOLED). In the AMOLEDs manufactured using the a-Si as a backplane, thin-film transistors (TFTs) embodied in an AMOLED panel have a problem with stability. Thus, a threshold voltage characteristic of each of the TFTs may vary as time passes. Also, in the AMOLEDs manufactured using the poly-Si or low temperature poly-Si (LTPS) as a backplane, TFTs embodied in an AMOLED panel have a problem with uniformity. Thus, a threshold voltage characteristic of each of the TFTs may change from one another according to the position where each TFT is located.
The change in the threshold voltage characteristic of each TFT embodied in the AMOLED is presented as dirt, referred to by the Japanese term mura, on the AMOLED panel. Thus, the change in the threshold voltage characteristic deteriorates the quality of an image displayed on the AMOLED panel and also shortens the life of the AMOLED panel.
To solve the above problems, the AMOLED is driven in accordance with a digital driving method that will be described hereinbelow. FIG. 1 illustrates the structure of a general organic light emitting pixel. FIG. 2 is a graph showing the characteristics of the voltage and current of the driving TFT of FIG. 1.
Referring to FIGS. 1 and 2, an organic light emitting pixel 10 includes a switching TFT 11, a storage capacitor 12, a driving TFT 13, and an organic light emitting diode (OLED) 14. The switching TFT 11 outputs a data signal input through a data line DL, or signal line, to the storage capacitor 12 in response to a scan signal input through a scan line SL. The storage capacitor 12 receives the data signal output from the switching TFT 11 and stores the received data signal.
The driving TFT 13 is turned on/off based on the voltage level of the data signal stored in the storage capacitor 12. When the driving TFT 13 is turned on, the driving TFT 13 supplies a voltage, or current, supplied from a voltage supply line to the OLED 14. Thus, the OLED 14 emits light in response to the supplied voltage or current.
As shown in FIG. 2, even when the characteristics of a voltage Vsignal and current IOLED of the driving TFT 13 vary according to position or time, if the AMOLED is driven in accordance with the digital driving method, the driving TFT 13 is simply used as a switch, so that there is not much change in the amount of current flowing to the OLED 14.
FIG. 3 illustrates a conventional digital driving method. For the convenience of explanation, FIG. 3 illustrates an example of the digital driving method to embody a total of sixteen gray values, in which a frame includes four sub-frames Sub-frame1 through Sub-frame4. In this example, the frame is referred to as a field and the sub-frame is referred to as a sub-field.
As shown in FIG. 3, a data signal used simply to turn on/off the driving TFT 13 at each sub-frame Sub-frame1 through Sub-frame4 is stored in the storage capacitor 12 shown in FIG. 1. Also, the gray value or gradation of the OLED 14 at each sub-frame Sub-frame1 through Sub-frame4 is presented as an integration value of the current supplied to the OLED 14 through the driving TFT 13 that is turned on.
For example, the OLED at a first row emits light for 8T during the first sub-frame Sub-frame1, for a time 4T during the second sub-frame Sub-frame2, for a time 2T during the third sub-frame Sub-frame3, and for a time 1T during the fourth sub-frame Sub-frame4. In this example, the time T indicates the time during which the driving TFT 13 is turned on. Thus, the OLED at the first row can present value Gray 16.
The OLED at the second row that does not emit light during the first through fourth sub-frames Sub-frame1 through Sub-frame 4 can present value Gray 0. Also, the OLED at the third row that emits light only during the third and fourth sub-frames Sub-frame3 and Sub-frame 4 can present value Gray 4. The OLED at the fourth row that emits light only during the first, third, and fourth sub-frames Sub-frame1, Sub-frame3, and Sub-frame 4 can present value Gray 12.
As shown in FIG. 3, when one frame is formed of four sub-frames, due to the characteristic of a digital driving method, the driving TFT 13 needs to supply a large amount of current at a fast frequency to the OLED 14 during a single frame. For example, the driving TFT 13 at the first row supplies a large amount of current to the OLED 14 four times and the driving TFT 13 at the fourth row supplies a large amount of current to the OLED 14 three times.
When a single frame is formed of n number of sub-frames, where n is a natural number, the driving TFT 13 needs to supply a large amount of current to the OLED 14 a maximum n times due to the characteristic of the digital driving method. Thus, as lots of stress is applied to the OLED 14, the function of the OLED 14 is rapidly degraded and a change in the amount of current flowing in the OLED 14 occurs as time passes. Thus, the change in the amount of current reduces the brightness of the AMOLED panel including the organic light emitting pixel 10 and shortens the life of the AMOLED.
Therefore, the structure of a light emitting pixel that is completely independent of the deviation of each of the driving TFTs embodied in the AMOLED panel and that can supply a constant amount of current to the OLED regardless of the deterioration of the function of the OLED that is generated as time passes, and a method for driving the light emitting pixel, are needed.